“If we can get more people taking an interest and submitting records, so much the better,” says Dave Kilbey, director of Natural Apptitude, which designed and launched the app.

“Hopefully it will mean people will become a bit more aware of the problems, and what to look for.”

So far the results show a particular concentration of knotweed in South Wales, the Midlands, London, Scotland’s central belt and Cornwall – where the plant was also introduced by Victorians into ornamental gardens.

Those looking for a property can use the app to find out if knotweed has been found nearby – but the fact it is not on the map does not mean it is not present; it is simply that no one has reported it.

How to recognise Japanese knotweed

Image caption
The plant was first imported to the UK in the 19th century

Dense thickets of green, purple-speckled, bamboo-like stems up to three meters tall

Heart or shield-shaped leaves

Alternate leafing pattern along stems

Completely hollow stems that can be snapped easily

Tiny creamy white flowers August to October

Rivers and canals

The data provided by the PlantTracker app is also added to the National Biodiversity Network (NBN) atlas, which aims to track the whereabouts of all the UK’s plants and animals, from bee orchids to goshawks.

Even though it has only been available to the public since April, and is not yet fully functional, the atlas has further information about Japanese knotweed locations.

The map shows more than 43,000 historical records for the plant, going back to 1900.

But Purba Choudhury, communications officer for the NBN, says that if there are no records in your area, that doesn’t guarantee its absence.

“Conversely, the record you are seeing might be an old record, and the Japanese knotweed might have been removed since the record was uploaded,” she says.

Image copyrightNBN atlas

Image caption
Certain areas of the UK are worst affected, such as South Wales